1. Field of the Invention
The present invention relates to an optical transmission system of the wavelength division multiplexing (WDM) method (which will be referred to hereinafter as a WDM transmission system) for transmitting signals of multiple channels (of respective wavelengths different from each other), having an optical amplifier, and to a channel assigning method of determining channels to be added or dropped at respective nodes.
2. Related Background Art
From the social needs with arrival of advanced information society, research and development has actively been conducted on long-haul optical communications including large-capacity and high-speed optical communications such as picture communications or the like, internal communications, etc. using optical fiber transmission networks. Here the WDM transmission systems are systems that implement large-capacity and high-speed optical communications by transmitting signals of multiple channels (which are signals of mutually different wavelengths and which will be referred to hereinafter as WDM signals) through optical fiber lines, and they have been developed and introduced so as to be ready for quickly increasing demands for communications making use of the internet and others in recent years.
In such WDM transmission systems, an optical amplifier, such as an Er(erbium)-doped fiber amplifier (EDFA: Erbium-Doped Fiber Amplifier), is utilized in order to compensate for transmission losses produced during signal transmission over long distance in the multiple channels. The optical amplifier is an optical device that supplies pumping light of a predetermined wavelength to an optical waveguide doped with such a rare earth as Tm, Pr, Nd, or the like, as well as above Er, (for example, an Er-doped optical fiber, EDF: Erbium-Doped Fiber) to amplify signals entering this optical waveguide. Semiconductor optical amplifiers are also employed as such optical amplifiers.
The inventors have investigated the prior arts described above and found the following problems. As the WDM transmission systems there are multidrop systems that transmit the WDM signals of mutually different wavelengths in an optical transmission line while adding or dropping part of the WDM signals at every node (signal add point or signal drop point) located on the optical transmission line. The conventional phototransmission systems of the multidrop type and assignment of signal channels to be added or dropped are described, for example, in Japanese Patent Applications Laid-Open No. H07-177097, Laid-Open No. H07-231300, and so on.
When the optical amplifier is set on the transmission line of the WDM transmission system of this multidrop type, a plurality of signal branchpoints are installed on the transmission line located on the output side of the optical amplifier. At each signal branchpoint, part of the WDM signals amplified by the optical amplifier (e.g., a signal of a channel to be dropped) is branched into a branch line. At this time, attenuation of each WDM signal propagating in the transmission line is dependent upon a transmission distance and attenuation values of the respective WDM signals differ from each other, depending upon installation locations of the respective signal branchpoints from the optical amplifier.
Here an amplification gain of the optical amplifier demonstrates wavelength dependence in general. If the gain is controlled so as to keep power levels of output signals constant (constant output), there will be a possibility of causing dispersion of power levels of signals dropped at the respective signal branchpoints, because of the aforementioned difference between the transmission losses of near signal branchpoints and far signal branchpoints from the optical amplifier.
On the other hand, it is also possible to reduce the dispersion of power levels of signals dropped at the signal branchpoints, by controlling the amplification gain of the optical amplifier so that a certain slope is preliminarily given to a gain curve demonstrating the wavelength dependence of gain. However, such control of amplification gain will complicate the device configuration of the optical amplifier and increase manufacturing cost of the optical amplifier.
The present invention has been accomplished in order to solve the above problems and an object of the present invention is to provide an optical transmission system provided with a structure for reducing the dispersion of power levels of signals dropped at the respective signal branchpoints located on the downstream transmission line from the optical amplifier, and also to provide a channel assigning method of assigning channels to be dropped, to the respective signal branchpoints.
An optical transmission system according to the present invention comprises an optical transmission line in which signals of multiple channels of mutually different wavelengths included in a predetermined wavelength band propagate, an optical amplifier disposed on the transmission line and having a gain depending on wavelength, and a plurality of signal branchpoints disposed on the output terminal side of the optical amplifier in the transmission line. Particularly, the optical transmission system according to the present invention is characterized in that each of the signal branchpoints drops a signal of a channel with a lowest power level out of the signals of the multiple channels arriving thereat, in order to accomplish the above object. A channel assigning method of the present invention for assigning channels to be dropped, to respective signal branchpoints, comprises steps of successively selecting one of the signal branchpoints, specifying a channel with a lowest power level out of the channels, for every signal branchpoint thus selected, and preliminarily or dynamically assigning the channel thus specified, to the selected signal branchpoint, as a channel of a signal to be dropped.
The inventors focused attention on the fact that the gain of the optical amplifier such as the EDFA applied to the optical transmission systems had the wavelength dependence, and discovered that it was possible to reduce the dispersion of power levels of dropped signals at the respective signal branchpoints placed on the transmission line in which the amplified signals propagated, by making use of correlation between transmission distances and the gain curve demonstrating the wavelength dependence. Namely, as the distance of a signal branchpoint increases from the optical amplifier, the power level of a signal to be dropped at the signal branchpoint becomes lower because of the transmission loss. To overcome it, the optical transmission system and the channel assigning method according to the present invention are arranged to set or select a wavelength of a signal with a lowest power level immediately before dropping out of signals arriving at each signal branchpoint after passage through other signal branchpoints present between the signal branchpoint of interest and the optical amplifier, as a wavelength of a signal to be dropped at the signal branchpoint.
Since the signals of the channels amplified by the optical amplifier uniformly attenuate while propagating in the transmission line on the output terminal side of the optical amplifier, the signals are dropped in order from the output signal with the lowest power level at the output terminal of the optical amplifier. This means that an output signal with a higher power level is set to have a longer transmission distance before dropping (or to have a greater transmission loss). This reduces the dispersion of signal power levels among dropped channels at low cost and readily with making use of the gain curve of the optical amplifier as it is, and without complicating the structure of the optical transmission system and the device configuration of the optical amplifier. In the case wherein signals of plural channels are dropped at one signal branchpoint, the system and method can be arranged so that one of the channels to be dropped is the one with the lowest signal power level as described above.
It is preferable here that each signal branchpoint include either of an optical ADM (Add-Drop Multiplexer) and a WDM coupler. The optical ADM is a branching device with excellent wavelength selectivity, which can adequately suppress crosstalk between signal channels, even if wavelength spacings are narrow between signal channels. The WDM coupler is compact, inexpensive, and easy to handle, and less attenuates signals.
Further, each signal branchpoint may include an optical coupler and a band-pass filter. The optical coupler is compact, inexpensive, and easy to handle and thus is effective to such circumstances that the number of signal channels is not so large and the system configuration is desired to be especially simple. When the band-pass filter is of a variable type, degrees of freedom can be increased for modification of the system.
Particularly, when each signal branchpoint includes a variable band-pass filter, it becomes feasible to select branch channels at respective signal branchpoints, or to centralize assignment of branch channels to the respective signal branchpoints (or implement dynamic assignment of branch channels).
When a branch channel is selected at each signal branchpoint, the signal branchpoint is preferably provided with a local control system. This local control system monitors power levels of signals arriving in respective channels and then specifies a channel with a lowest power level out of the channels, as a channel of a signal to be dropped thereat.
When the branch channels are assigned to the respective signal branchpoints in centralized manner, the optical transmission system is preferably provided with a centralized control system for assigning channels to be dropped, to the respective signal branchpoints while monitoring power levels of signals arriving in respective channels at the respective signal branchpoints. This centralized control system selects a signal branchpoint as a control objective out of a plurality of signal branchpoints and specifies, for every signal branchpoint selected, a channel with a lowest power level out of the channels, as a channel of a signal to be dropped at the signal branchpoint selected.
The optical amplifier is preferably an Er-doped fiber amplifier (EDFA). Since the channel assigning method according to the present invention makes use of the wavelength dependence without provision of a device configuration for flattening the gain curve of the EDFA, it becomes feasible to reduce the dispersion of power levels of signals dropped at the respective signal branchpoints while adequately controlling the manufacturing cost in a reasonable range. The optical amplifiers except for the above can also be applied similarly to the optical transmission system and channel assigning method according to the present invention.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.